1.使用Java的输入、输出流将一个文本文件的内容按行读出，每读出一行就顺序添加行号，并写入到另一个文件中。

在Java中，你可以使用FileInputStream和FileOutputStream来处理文本文件的输入和输出，`BufferedReader`用于逐行读取，`PrintWriter`或`BufferedWriter`用于按行写入并添加行号。以下是简单的步骤：

1. 首先，你需要导入必要的包：

import java.io.*;

2. 定义两个File对象，分别表示源文件和目标文件：

File inputFile = new File("source.txt");
File outputFile = new File("output.txt");

3. 创建输入和输出流对象，并打开它们：

try (BufferedReader reader = new BufferedReader(new FileReader(inputFile));
     BufferedWriter writer = new BufferedWriter(new FileWriter(outputFile))) {
    //...
} catch (IOException e) {
    System.err.println("Error reading or writing file: " + e.getMessage());
}

4. 使用`BufferedReader`逐行读取，每读取一行就添加行号：

int lineNumber = 1;
String line;
while ((line = reader.readLine()) != null) {
    writer.write(lineNumber++ + ": " + line);
    writer.newLine(); // 添加换行符
}

5. 最后记得关闭流以释放资源：

writer.close();

完整代码示例：

import java.io.*;

public class Main {
    public static void main(String[] args) {
        File inputFile = new File("source.txt");
        File outputFile = new File("output.txt");

        try (BufferedReader reader = new BufferedReader(new FileReader(inputFile));
             BufferedWriter writer = new BufferedWriter(new FileWriter(outputFile))) {

            int lineNumber = 1;
            String line;
            while ((line = reader.readLine()) != null) {
                writer.write(lineNumber++ + ": " + line);
                writer.newLine();
            }

        } catch (IOException e) {
            System.err.println("Error reading or writing file: " + e.getMessage());
        }
    }
}